This project will examine the intracellular transport of two glycoproteins, immunoglobulins (Ig) and MHC class I antigens, in murine B lymphocytes. Proper intracellular transport of these proteins to the plasma membrane is cricial for the functioning of B lymphocytes as Ig secretors and as antigen presenters. Two stages of transport are of special interest: 1. The transition between the Endoplasmic Reticulum (ER) and the Golgi complex. 2. The transition between the Golgi complex and the cell surface. Our main question about the ER-Golgi transition is identifying those structural features of IG lambda chains which are crucial for executing this rate-limiting step of transport. Such features may in fact serve as transport signals, for example by mediating interactions of lambda with other cellular proteins. To identify and map such structural features, we shall construct lambda transport mutants and ask that they satisfy two requirements: that they fail to exit the ER and that they fold properly. Such mutants can reasonably be interpreted as defective in transport signals. These mutants will then be used to examine the role of resident ER proteins, like Binding Protein, in retaining non-secreted Ig in the ER. Understanding the transition between the Golgi complex and the cell surface is important with respect to Beta cell function in antigen presentation, since this is a stage where endocytosed ligands and receptors mix with the exocytic traffic. We observed that the protonophore CCCP perturbs transport through this stage, in a compartment distal to the Golgi stack, which we think is the trans-Golgi reticulum. We propose to characterize this compartment further, by immuno-electron microscopy and histochemistry. Moreover, we propose to isolate the trans-Golgi reticulum and study its biochemical properties. Finally, the transport of MHC class I antigens between the Golgi complex and the cell surface will be studied, by examining the phosphorylation of these molecules and the relation between phosphorylation and the traffic to and from the cell surface.